Polytrimethylene terephthalate resins with improved properties

ABSTRACT

A composition comprising Polytrimethylene terephthalate (PTT) with a reduced emission of acrolein is obtained by contacting Polytrimethylene terephthalate (PTT) resin with an effective amount of a melt stable, organic stabilizing compound. Such PTT resin has an acrolein formation rate at 280° C. of less than 15 ppm/minute. Preferably the residual acrolein content of the PTT resin is less than 10 ppm. The additive organic stabilizing compounds are free from nitrogen atoms. Preferred additive compounds include polyfunctional alkohols, alcoholates, aromatic carboxylic acid anhydrides, carboxylic acids and salts of carboxylic acids. The additive compounds are added to the polymer in molten state and/or to the resin, which is then processed to fibers, filaments, non-wovens, films and/or mouldings.

The present invention relates to polytrimethylene terephthalate (PTT)resins having a low contaminants formation rate and a low content ofresidual contaminants, to a method for the production of such resins andto articles obtained from said resins.

In general the polytrimethylene terephthalate resins used for theproduction of filaments, fibers and spin-fleeces for carpets, textilegoods (wear, room textiles, technical textiles), further for theproduction of films and plastic materials have intrinsic viscositiesusually above 0.75 dl/g. Resins with these values are obtained either byan all-melt polycondensation process of terephthalic acid ordimethylterephthalate with propanediol or by a two step process of meltand solid state polycondensation process, whereas in the firstpolycondensation reaction the intrinsic viscosity generally does notreach values higher than 0.6-0.7 dl/g and the resin viscosity has to beincreased by solid-state polycondensation (SSP).

The all-melt process e.g. is performed as set forth in U.S. Pat. No.6,277,947, a melt plus SSP process e.g. is disclosed in WO 97/23543.

The SSP reaction is performed by heating the pellets of the resin, afterits crystallization, to a temperature from about 180° to 230° C. in astream of inert gas (nitrogen or others) for a time sufficient to obtainthe intended IV value.

Extrusion of the resin from the all-melt processes to obtain the pelletsto be subjected to SSP is generally performed in air. Resin processingto obtain the various articles is also performed in air.

During resin pelletizing and treatment steps such as extrusion of thechips to spin filaments, fibers and fleeces for carpets, textile goods(wear, room textiles, technical textiles) and further to produce filmsand plastic materials, contaminants are generated in considerableamounts.

Amongst the contaminants acrolein plays an important role. The residualacrolein content of the products can exceed up to 50 ppm and theacrolein formation rate, measured at 280° C., can be higher than 30ppm/minute.

The presence of acrolein, like the presence of other contaminants,negatively affects the application of PTT products, specifically inpresence of temperatures higher than 60° C.

Accordingly, it is desirable to have the lowest possible acroleincontent in the resin that is used in human environment.

It is also desirable for articles which do not make contact with wearand room textiles to have a low residual acrolein content.

In order to obtain articles having a low residual acrolein content, theinitial resin must be characterized not only by a low acrolein contentbut also and preferably by a low rate of acrolein formation at theprocess temperatures used to prepare the articles and at the thermalconditions of their application, i.e. during concentrated exposure tothe sun's rays, ironing or drying processes.

Methods are known for obtaining polytrimethylene terephthalate resinswith a low acrolein formation rate. It was found that PTT thermalstabilizers and antioxidants reduce the formation of acrolein in PTTwhen heated/aged in air as disclosed by U.S. Pat. No. 6,093,786 and U.S.Pat. No. 6,242,558. But PTT comprising these additives is deemed toinvolve the drawback of showing a considerable residual acroleinformation rate at ambient temperatures.

EP-A 1 142 955 discloses a titanium oxide-containing PTT resincomposition. A PTT containing titanium oxide has the problem that thePTT forms large amounts of acrolein and allyl alcohol in comparison withPTT without titanium oxide. Therefore, in EP-A 1 142 955 thepolycondensation is carried out in presence of a phosphorous compoundand/or a hindered phenol in order to inhibit the generation of byproducts such as acrolein and allyl alcohol.

The generation of dipropylene glycol as a pre product of acrolein duringPTT fabrication can be reduced by addition of basic metal salts to thereaction mixture before or during the polycondensation (U.S. 2002/010310A1).

In WO 01/81447 a PTT polymer is disclosed with reduced amounts ofdipropylene glycol monomer units. This composition is obtained withoutaddition of any stabilizers and when heated in air, it has a reducedtendency to generate acrolein.

WO 00/58393 discloses melt stable aliphatic and aromatic amines andamides as stabilizing additives for 3-hydroxypropanoxy terminatedpolymers which reduce the emission of acrolein from such polymers (e.g.PTT).

Surprisingly it has been found that the emission of acrolein from aPolytrimethylene terephthalate (PTT) resin can be reduced by acomposition comprising Polytrimethylene terephthalate (PTT), wherein thecomposition is obtainable by contacting molten Polytrimethyleneterephthalate (PTT) resin with an effective amount of a melt stable,organic stabilizing compound. Such compound is selected from the groupcomprising polyfunctional alkohols, alcoholates, anhydrides ofcarboxylic acids, carboxylic acids and their salts, carbohydrates andderivatives of carbohydrates.

A melt stable, organic stabilizing compound in the idea of the presentinvention does not contain any nitrogen atoms, in contrast to the meltstable nitrogen containing compounds disclosed in WO 00/58393.

The term melt stable means that said organic stabilizing compound doesnot decompose at usual temperatures of a PTT melt during processing ofabout 250-280° C. The stability of said compounds is not determined inthe PTT melt itself but by usual analytical methods (e.g. kofler hotbench). Such methods and physical data of said compounds are generallyknown from literature (e.g. Handbook of Chemistry and Physics, 83^(th)edition, CRC Press, Boca Raton, 2002).

According to the invention there is provided for the first time apolytrimethylene terephthalate (PTT) resin composition having anacrolein formation rate at 280° C. of less than 15 ppm/minute,preferably lower than 1 ppm/minute and particularly lower than 0.1ppm/min. The PTT resin composition of the invention advantageously ischaracterized by a low acrolein content <10 ppm, preferred <1 ppm andmost preferred <0.1 ppm.

The said compound can be added to the PTT either when the PTT is in themolten state or when the PTT is in the solid state, provided that thesolid state PTT is processed thereafter through the molten state,thereby allowing for a thorough mixing of PTT and additive.

The amount of compound to be added to the PTT may vary over a broadrange. It is preferred, however, that the compound is added in an amountof 10-30.000 ppm based on the total weight of the PTT.

The behavior of the polytrimethylene terephthalate resins comprisingsubstances according to the present invention is unusual with respect tothe formation of acrolein, since the compounds usually added to PTT asstabilizers and antioxidants do not result in a limitation of theacrolein content or the formation rate of acrolein.

It is known that thermal stabilizers and antioxidants in an amount inthe range from 0.1 to 1% by weight are added to the Polytrimethyleneterephthalate resin during the preparation of said resin in theesterification or transesterification step or in the subsequent step ofpolycondensation in the melt. Widely used thermal stabilizers for thepurpose of stabilization embrace e.g. phosphorous compounds. Widely usedantioxidants encompass e.g. hindered phenols.

It is known from U.S. Pat. No. 6,093,786 and U.S. Pat. No. 6,242,558that the generation of acrolein in the destillate streams during thepolymerization of PTT is reduced when an organo-phosphite thermalstabilizer is added during the reaction and also that the addition of ahindered phenolic antioxidant to PTT results in a polymer which is lowerin color and less likely to generate acrolein when heated/aged in air.The resulting PTT resins have acrolein concentrations of >10 ppm in thePTT.

U.S. Pat. No. 6,326,456 discloses phosphorous compounds, hinderedphenolic esters and organophosphites as stabilizers which stop thecatalysis of the thermal degradation of PTT melt by ions of heavymetals.

Using the compounds in accordance with the present invention inconcentrations of 10 to 30.000 ppm related to the polymer theconcentration of acrolein in the pellets can be reduced to <10 ppmpreferably <1 ppm and most preferred <0.1 ppm.

The compounds preferably are added to the PTT melt before pelletizing.These compounds reduce the content of acrolein in the PTT afterre-extrusion of the chips and spinning to fibers or processing to otherapplication products. It was found that the effect of the said compoundscontinued also under solid conditions and the acrolein content in thechips, fibers and other application products still decreased.

Heating the PTT products the acrolein formation rate dramatically islowered as well.

It was found that melt stable polyfunctional alkohols and alcoholatesare capable of reducing the emission of acrolein. Preferred examples ofthese compounds are selected from the group consisting oftrishydroxy-methylpropane, pentaerythrite, sorbitol, polyvinylalkoholand aluminium-methanolate.

In another embodiment the compound to be added is selected from thegroup consisting of anhydrides of carboxylic acids, carboxylic acids andsalts of carboxylic acids. Preferred anhydrides are anhydrides ofaromatic carboxylic acids. Preferred examples of such compounds areselected from the group consisting of phthalic anhydride, isophthalicanhydride and pyromellitic dianhydride. A preferred salt of a carboxylicacid is zinc-acetate.

It was also found that carbohydrates and derivatives of carbohydratescan surprisingly reduce the emission of acrolein.

It should be noted that all of the above additives can be used singly orin a mixture of two or more compounds.

In this connection it was surprisingly found that a combination of analkoholate and an anhydride of a carboxylic acid can synergisticallyreduce the emission of acrolein from a PTT resin. In a preferredembodiment, a combination of aluminium-methanolate and phthalicanhydride is added.

It was also found that a combination of an alkoholate and apolyfunctional alcohol can synergistically reduce the emission ofacrolein from a PTT resin. In a preferred embodiment, a combination ofaluminium-methanolate and pentaerythrite is added.

Said compounds preferably are added to the resin in the melt by mixingbefore pelletizing or in an extruder before forming moldings or fibersor before subjecting the resin to the SSP step.

The treatment of the resin in the molten state can be advantageouslyperformed in an inert-gas atmosphere (nitrogen), for example by passinga stream of inert gas flow in the environment in which preparation ofthe articles occurs. It is also possible to work in air, but lesssatisfactory results are obtained.

It has been found, and it is, a further aspect of the invention, that itis possible to avoid coloring problems due to the presence of thecompounds in accordance with the present invention in the melt or whenthe solid resin is subjected to SSP at a temperature above 180° C. orwhen the solid resin is re-extruded before forming moldings and fibersby using these compounds in combination with thermal stabilizers andantioxidants.

As already indicated, the antioxidants can be used in combination withthe said acrolein converting compounds can be used in the processaccording to the present invention.

The antioxidants preferably include phosphites having the formulaP(OR′)₃, whereas R are identical or different and are alkyl or arylgroups. It is also possible to use phosphates having the formulaOP(OR)₃, whereas R has the above mentioned meaning.

Diphosphite compounds such as “Ultranox 626” or “Ultranox 627” (Brandnames of compounds marketed by General Electric Specialty Chemicals) arepreferred.

Especially preferred are phosphonic acid derivatives of the formulaZ-R—(PO)(OH)₂ with R═(CH₂)_(n) and n=1-6 and Z=COOH, OH or NH₂.

Other antioxidants which can be used are phenolic antioxidants, such ashindered phenolic compounds, for example the compound “Irganox 1010” or“Irganox 1076” by Ciba-Geigy or the compounds as discussed in U.S. Pat.No. 6,093,786 and U.S. Pat. No. 6,242,558.

The PTT can comprise up to 20% other monomer units as naphthalenedicarbonic acid or hydroxybenzoic acid or other diols as butanediol orethanediol or cyclohexanediol.

In accordance with the present invention also polymer blends up to 20wt.-% with other polyesters or combinations of them as polybutyleneterephthalate, polyethylene terephthalate, polyethylene naphthalateand/or copolyethylene terephthalate can be used as PTT resins.

Resins with a low acrolein formation rate and low acrolein contenthaving an intrinsic viscosity of more than 0.75 dl/g and being obtainedin an all-melt process or by additional solid stating are particularlysuitable for the production of filaments, fibers and spin-fleeces forcarpets, textile goods (wear, room textiles, technical textiles),further for the production of films and plastic materials and for anyother application which requires a low acrolein content and a lowacrolein formation rate.

The following examples are given by way of non-limitative illustrationof the invention.

Analytical Methods

Acrolein Content by Headspace-GC

1 g of PTT polymer which has been ground in liquid nitrogen is heated ina closed vial under nitrogen atmosphere at 150° C. for 30 minutes. Fromthe vapour space of the vial a sample is taken and transferred to a gaschromatograph equipped with a flame ionization detector for analysis ofthe acrolein content.

Acrolein Formation by Thermodesorption-GC

Prior to analysis PTT polymer is ground in liquid nitrogen. A sample of200 mg is heated in a nitrogen stream at 280° C. for 12 minutes. Thenitrogen-stream is passed through a trap filled with TENAX (reg. trademark) where the organic components are removed from the stream. Afterthe specified time the organic load is removed from the trap by rapidtemperature increase transferred by a nitrogen stream to a gaschromatograph (FID) where it is further analyzed.

Intrinsic Viscosity (IV)

125-145 mg polymer are weighed into a cylindrical dissolution vessel, 25ml phenol/1,2 dichlorbenzene (60:40 w.w.) is added. The sample isstirred 20 min. at 135° C. and cooled down. The IV of the solution ismeasured in an Ubbelohde viscosimeter at 25° C.

EXAMPLES

1. PTT Chips were dried (10 h, 130° C.), mixed with the powder of themelt stable organic compound capable of chemically reacting and therebyremoving acrolein, extruded by a Husmann Extruder Model ES 30,throughput 6 kg/ h at 260° C. The melt passed a 9 element mixing line(DN15 SMX, Sulzer) within 4 min. and was spun out to strand. The strandwas cooled by a 2 m long waterbath and granulated by a Scheer lab cutterto PTT chips. The chips were dried (10 h, 130° C.) and prepared toanalyze the acrolein content.

2. Comparison example, chip PTT chips without the additives of thepresent invention were re-extruded under the same conditions asdescribed in example 1 and prepared to analyze the acrolein content.

3. Re-extruded grounded PTT samples of pellets were filled in thedesorption apparatus to desorb and analyze the acrolein. The followingtable shows the results of the analysis: Acrolein Acrolein desorptiondesorption from from original reextruded Acrolein chips chip/fiberreduction Sample [ppm/12 min] [ppm/12 min] [%] Sample1 420 415 noneSample1 + 0.1% — 320 23 phthalic anhydride Sample1 + 0.1% — 162 61phthalic anhydride + 0.01% Al- methanolate

4. The PTT samples were analysed by the Headspace GC-Method to detectacrolein concentration in the reextruded polymer. The following tableshows the results: Acrolein concentration in the Acrolein reextrudedreduction related Sample chips [ppm] to sample1 [%] Sample1 26 noneSample1 + 0.1% 17 35 trishydroxy- methylpropane Sample1 + 0.1% 18 31D-sorbitol Sample1 + 0.1% 19 27 polyvinyl-alcohol Sample1 + 0.1% 20 23zincacetate Sample1 + 0.1% 18 31 phthalic anhydride Sample1 + 0.1% 18 31pentaerythrite Sample1 + 0.1% Al-  8 69 methanolate Sample1 + 0.01% 2023 Al-methanolate Sample1 + 0.1%  9 65 pentaerythrite + 0.01%Al-methanolate Sample1 + 0.1% 12 54 phthalic anhydride + 0.01%Al-methanolate

5. Repeating of Headspace GC-Method with some samples to analyzeacrolein in the PTT Chips after storage of the samples at 25° C.:Acrolein concentration Acrolein in the reduction reextruded relatedStorage chips to time after storage sample1 Sample [days] [ppm] [%]Sample1 0 26 none Sample1 7 26 none Sample1 + 0.1% 7 14 46 trishydroxy-methylpropane Sample1 + 0.1% 7 16 38 pentaerythrite Sample1 + 0.1% 7 1638 D-sorbitol Sample1 + 0.1% 7 18 31 polyvinyl- alcohol

1. A composition comprising polytrimethylene terephthalate (PTT),wherein the composition is obtainable by contacting moltenpolytrimethylene terephthalate (PTT) resin with a melt stable,stabilizing alcoholate compound to reduce the emission of acrolein fromsaid resin.
 2. The composition of claim 1, having an acrolein formationrate at 280° C. of less than 15 ppm/minute.
 3. The composition of claim2, wherein the acrolein formation rate at 280° C. is less than 1ppm/minute.
 4. The composition of claim 3, wherein the acroleinformation rate at 280° C. is less than 0.1 ppm/minute.
 5. Thecomposition of claim 1, wherein the residual acrolein content is lessthan 10 ppm.
 6. The composition of claim 5, wherein the residualacrolein content is less than 1 ppm.
 7. The composition of claim 6,wherein the residual acrolein content is less than 0.1 ppm.
 8. Thecomposition of claim 1, wherein the alcoholate is added in an amount of10-30.000 ppm based on the total weight of the PTT resin.
 9. Thecomposition of claim 1, wherein the alcoholate compound to be added isaluminium-methanolate.
 10. The composition according to claim 9 whereinin addition to alcoholate at least one of a melt stable organic compoundselected from the group consisting of poly-functional alcohols,anhydrides of carboxylic acids, carboxylic acids and their salts,carbohydrates and derivatives of carbohydrates is added.
 11. Thecomposition according to claim 10 wherein at least one melt stableorganic compound is added in an amount of 10-30.000 ppm based on thetotal weight of the PTT resin.
 12. The composition according to claim 10wherein the melt stable organic compound to be added istrishydroxy-methylpropane, pentaerythrite, sorbitol or polyvinylalcohol.13. The composition according to claim 10 wherein the organic compoundto be added is an anhydride of an aromatic carboxylic acid.
 14. Thecomposition according to claim 13 wherein the organic compound to beadded is phthalic anhydride, isophthalic anhydride or pyromelliticdianhydride.
 15. The composition according to claim 10 wherein theorganic compound to be added is zinc-acetate.
 16. The composition ofclaim 1, having an intrinsic viscosity of greater than 0.7 dl/g.
 17. Thecomposition of claim 1 containing up to 20% by weight of polyethyleneterephthalate, polyethylene naphthalate, polybutylene terephthalate orcopolyethylene terephthalate.
 18. The composition of claim 1, containingone or more of polymer stabilizers selected from the group consisting ofphosphorous acid, phosphoric acid, alkylesters of phosphoric acid,hydroxyalkylester of phosphoric acid, diphosphites and phosphonic acidsin a concentration of 1-200 ppm as phosphorus in the substances.
 19. Thecomposition of claim 18 wherein the phosphonic acid is selected from2-carboxyalkylphosphonic acid, 2-hydroxyalkylphosphonic acid,2-aminoalkylphosphonic acid and the alkyl group is selected from methyl,propyl, butyl, pentyl and hexyl.
 20. The composition of claim 1containing thermo-oxidative polymer stabilizers as phenolicantioxidants, such as hindered phenolic compounds in concentrations of0.1-3% by weight.
 21. The composition of claim 1 having been subjectedto a solid-state polycondensation reaction in an inert-gas atmosphere.22. A method for preparing the composition according to claim 1 whereinthe melt stable stabilizing alcoholate compound and optionally the atmelt stable organic compound selected from the group consisting ofpolyfunctional alcohols, anhydrides of carboxylic acids, carboxylicacids and their salts, carbohydrates and derivatives of carbohydratesand optionally said thermo-oxidative Polymer stabilizers and optionallyantioxidants are added to the polymer in molten state and optionally tothe resin, which is then processed to fibers, filaments, non-wovens,films or mouldings.
 23. An article of manufacture selected from fabrics,knittings and carpets made from filament yarns, staple fibers, bulkedcrimped yarns (BCF) and non-wovens comprising the composition of claim1.